A film capacitor is a type of capacitor which includes flexible resin films serving as a dielectric and metal films serving as first and second counter electrodes, the resin films being interposed between the metal films that face each other, and the metal films being arranged on both main surfaces of the resin films. The film capacitor usually has a cylindrical shape obtained by winding the dielectric resin films. First and second external terminal electrodes are formed on opposing first and second end faces of the cylinder. The first counter electrode is electrically connected to the first external terminal electrode. The second counter electrode is electrically connected to the second external terminal electrode.
In such a film capacitor, films mainly composed of polypropylene are commonly used as the dielectric resin films from the viewpoint of electrical characteristics, formability, and handleability, as described in, for example, Japanese Unexamined Patent Application Publication No. 10-119127 (PTL 1).
However, where a thermoplastic resin, such as polypropylene, is used as a main component for the dielectric resin films, there is a problem of heat resistance. That is, such dielectric resin films are not suitable for high temperature applications, for example, use in engine compartments of automobiles.
Furthermore, it is necessary to reduce the thickness of the layer of a dielectric resin film to satisfy the recent demand for miniaturization from the market. A film mainly composed of polypropylene is produced through steps of thermally melting a material and drawing the melted material into a sheet. For example, it is disadvantageously difficult to form a sheet having a stable thickness at a thickness of less than 2.5 μm, because the degree of technical difficulty is increased.
International Publication No. 2006/100833 (PTL 2) teaches the use of polyisocyanate-crosslinked polyvinyl acetal as the main component for the dielectric resin films in place of polypropylene and that a coupling agent surface treated high dielectric constant filler is added in order to increase the dielectric constant of the resin. Polyvinyl acetal is thermosetting and can be dissolved in a solvent. So, unlike polypropylene, a thin-layer sheet can be formed without thermal melting or drawing.
However, variations of properties over the same film are likely to be relatively large for the dielectric resin films disclosed in PTL 2. Thus, even if the dielectric resin films as disclosed in PTL 2 are used, a guaranteed heat resistance for the same production lot, i.e., a guaranteed withstand voltage, is disadvantageously reduced at a specific temperature.